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Cardio is the cornerstone to any good weight-loss plan—especially intervals, where you alternate going as fast as possible with brief periods of recovery. To help you make those sprints as effective as possible, we asked Phil Campbell, an interval-training expert and author of Ready, Set, Go! Fitness for his best fat-burning tips.

1/ Warm-up

"You want to take your heart rate up progressively," Campbell says, "Focus on large muscles like the hamstrings and quads. Try ankle circles, knee circles, some light hip stretches—they all get your muscles and ligaments prepared to fire faster."

2/ Time Your Sprints

"The key is to get totally winded in 30 seconds or less," he says, "If you're on a treadmill [or an exercise bike], start sprinting as the machine speeds up. When it reaches top speed, go all out for 30 seconds. Then slow it down and go nice and easy for 90 seconds to recover." If you're running outside, try to cover about 200 feet—roughly half a city block—as fast as possible. Walk slowly for 90 seconds to cool down, then repeat your course.

3/ Straighten Up

Almost everyone leans forward when they work out on a cardio machine. This lets you use gravity to pull you forward, making the effort a bit easier. "If you stay upright during your sprints," Campbell says, "it keeps the intensity on your muscle the entire time."